eagerly.
https://bugs.webkit.org/show_bug.cgi?id=65382

Reviewed by Oliver Hunt.

This introduces a memory reuse model similar to the one in FastMalloc.
A periodic scavenger thread runs in the background and returns half the
free memory to the OS on each timer fire.  New block allocations first
attempt to get the memory from the collector's internal pool, reverting
to OS allocation only when this pool is empty.

* heap/Heap.cpp:
(JSC::Heap::Heap):
(JSC::Heap::~Heap):
(JSC::Heap::destroy):
(JSC::Heap::waitForRelativeTimeWhileHoldingLock):
(JSC::Heap::waitForRelativeTime):
(JSC::Heap::blockFreeingThreadStartFunc):
(JSC::Heap::blockFreeingThreadMain):
(JSC::Heap::allocateBlock):
(JSC::Heap::freeBlocks):
(JSC::Heap::releaseFreeBlocks):
* heap/Heap.h:
* heap/MarkedBlock.cpp:
(JSC::MarkedBlock::destroy):
(JSC::MarkedBlock::MarkedBlock):
(JSC::MarkedBlock::initForCellSize):
(JSC::MarkedBlock::reset):
* heap/MarkedBlock.h:
* wtf/Platform.h:


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@92084 268f45cc-cd09-0410-ab3c-d52691b4dbfc